Method for metallization of a semiconductor device

ABSTRACT

A method for metallization of a semiconductor device. This method includes a) metallizing a set of collection fingers with a low-temperature serigraphy paste on at least a front surface of the semiconductor device, b) sintering, at a temperature below a temperature that would damage the semiconductor device, the serigraphy paste forming the set of metallized collection fingers, by performing a pressing operation on the collection fingers with a press, and c) metallizing at least one collection bus on the set of metallized collection fingers, electrically connecting the collection fingers to one another, with a low-temperature serigraphy paste.

TECHNICAL FIELD

This invention relates to a metallization method specifically adapted toso-called “low-temperature” methods for producing semiconductor devices.Such a method is particularly suitable for metallizing a heterojunctionsolar cell.

PRIOR ART

The principle of the amorphous/crystalline heterojunction is known andhas been in the public domain for ten years. Solar cells applying thisprinciple have already been patented.

The principle of this type of cell is to use a crystalline semiconductorsubstrate with a first type of conductivity. An amorphous semiconductorlayer with a second type of conductivity, opposite the first type ofconductivity, is deposited on one of the surfaces of the crystallinesubstrate. A PN junction is thus obtained, and is called aheterojunction because the two semiconductors used have different atomiccompositions and do not have the same forbidden bandwidth. It is thensimply necessary to produce a transparent electrode on a first surfaceof the junction and, on a second surface opposite this first surface, toproduce an ohmic contact electrode in order to obtain a heterojunctionsolar cell.

The U.S. Pat. No. 5,066,340 describes a heterojunction solar cell. Itcomprises a PN junction formed by a crystalline silicon substrate with afirst type of conductivity and an amorphous silicon layer with a secondtype of conductivity, opposite the first type of conductivity, producedon one of the surfaces of the crystalline substrate. This cell alsointegrates, between the crystalline substrate and the amorphous siliconlayer, an intrinsic microcrystalline silicon layer.

The U.S. Pat. No. 5,213,628 also describes a heterojunction solar cell.As in the U.S. Pat. No. 5,066,340, this cell comprises a heterojunctionformed by a crystalline silicon substrate with a first type ofconductivity and an amorphous silicon layer with a second type ofconductivity, opposite the first type of conductivity, produced on oneof the surfaces of the crystalline substrate. This cell integrates,between the crystalline substrate and the amorphous silicon layer, anintrinsic microcrystalline silicon layer.

The U.S. Pat. No. 6,091,019 describes a heterojunction cell. On a firstsurface of a crystalline silicon substrate with a first type ofconductivity, a plurality of successive deposits are provided so as toform a stack consisting of a plurality of layers: first, an intrinsicamorphous silicon laver, then an amorphous silicon layer doped with asecond type of conductivity opposite the first type of conductivity,then a conductive transparent oxide layer, for example indium and tinoxide (known as ITO for Indium Tin Oxide), and finally the metallizationof silver collection fingers. On a second surface opposite the firstsurface of the crystalline silicon substrate, the deposits are identicalexcept for the second layer, which is amorphous silicon doped with thefirst type of conductivity. Collection buses are then deposited on themetallizations produced on the two surfaces of the solar cell.

In this type of cell, the metallizations are performed by serigraphy,and must then be annealed. So as not to damage the amorphous siliconlayers, the metallizations must be annealed at “low temperature”, i.e.at a temperature below around 400° C. This heat treatment is necessaryto cause the metal to penetrate the silicon. This condition involves theuse of specific so-called “low-temperature” serigraphy pastes, forexample based on polymer/silver. The devices that do not compriseamorphous silicon or materials sensitive to a temperature above 400° C.preferably use so-called “high-temperature” serigraphy pastes, whichmust be annealed at around 800° C. This heat treatment makes it possiblefor the metal to penetrate the silicon, so as to ensure good contactwith the cell, but also to improve the resistivity of the serigraphypaste.

The major disadvantage of so-called “low-temperature” serigraphy pastesis that they have a resistivity ten times higher than the so-called“high-temperature” serigraphy pastes, used in particular in theproduction of homojunction solar cells. This high resistivity increasesthe series resistance of the heterojunction cells, causing a reductionin the peaking factor. The peaking factor is the ratio between theproduct of the maximum output voltage with the maximum output intensityand the product of the open circuit voltage with the short circuitcurrent intensity. This reduction in the peaking factor causes areduction in the efficiency of the solar cells.

In addition, the adhesion of these so-called “low-temperature”serigraphy pastes to the semiconductor devices is not alwayssatisfactory. This poor adhesion then presents problems in theinterconnection of devices by welding on the collection buses.

DESCRIPTION OF THE INVENTION

This invention is intended to propose a method for metallizing asemiconductor device making it possible to reduce the disadvantagesmentioned above, i.e. to reduce the resistivity of the serigraphy pasteused to metallize the collection fingers of the semiconductor device,and to improve the adhesion of this serigraphy paste on thesemiconductor device.

To achieve these objectives, this invention proposes a method formetallization of a semiconductor device including the following steps:

a) metallizing a set of collection fingers with a so-called“low-temperature” serigraphy paste on at least one surface, called the“front surface” of the semiconductor device, b) sintering, at atemperature below a temperature that would damage the semiconductordevice, of the serigraphy paste forming the set of metallized collectionfingers, by performing a pressing operation on these collection fingerswith a press,

c) metallizing at least one collection bus on the set of metallizedcollection fingers, electrically connecting the collection fingers toone another, with a so-called “low-temperature” serigraphy paste.

Thus, in the metallization of a semiconductor device, instead of firstperforming the metallization of the collection fingers, then themetallization of the collection bus, a sintering operation is insertedbetween these two steps, which sintering operation is performed at atemperature below a temperature that would damage the semiconductordevice, on the serigraphy paste forming metallized collection fingers,by pressing these collection fingers. This sintering makes it possibleto reduce the resistivity and to improve the weldability of the pasteforming the metallized collection fingers.

This metallization is advantageously applied to semiconductor deviceswith a heterojunction.

The method can include a step after step c), consisting of sintering, ata temperature below a temperature that would damage the semiconductordevice, the serigraphy paste forming the collection bus by a pressingoperation on said collection bus performed by the press. This pressingstep makes it possible to reduce the resistivity of the serigraphy pasteforming the collection bus.

The method can also comprise a step consisting of sintering, at atemperature below a temperature that would damage the semiconductordevice, a metallization located on a surface of the semiconductor deviceopposite the front surface by a pressing operation on said metallizationperformed by the press. This pressing step makes it possible to reducethe resistivity of the serigraphy paste forming the metallization.

The press used is, for example, a hydraulic or a pneumatic press.

The pressing is preferably performed at a temperature between aroundroom temperature and 400° C. The temperature of 400° C. is approximatelythe maximum temperature because, at a higher temperature, there would bedamage to the amorphous semiconductor.

It is preferable for the pressing to be performed at a pressure levelbetween around 10⁶ Pa and 2×10⁸ Pa, making it possible to sinter thecollection fingers.

During the pressing, the semiconductor device can be placed between thepress and a support. Means for protecting the semiconductor device canbe inserted between the semiconductor device and the press, and betweenthe semiconductor device and the support.

In this case, the protective means are preferably polyethyleneterephthalate films.

Means for uniform pressing can also be inserted between thesemiconductor device and the press.

In this case, the uniform pressing means are preferably a damper, forexample made of rubber or a plastic material, and a plate, for examplemade of silicon.

The collection fingers are preferably arranged so as to be parallel toone another.

It is possible for the collection fingers to be regularly spaced apartfrom one another. This arrangement makes it possible to obtain ahomogeneous collection of the current.

It is preferable for the collection bus to be arranged so as to besubstantially perpendicular to the set of collection fingers.

The metallization of the set of collection fingers and the metallizationof the collection bus can be performed by serigraphy.

The collection fingers and the collection bus are preferably producedwith an aluminium-based material or a noble metal such as silver.

This invention also relates to a semiconductor device comprisingcollection fingers and at least one collection bus, of which thecollection fingers and the collection bus can be made according to themethod described above.

It is preferable for the collection fingers of such a device to have awidth of around 100 micrometers and a thickness between around 20micrometers and 40 micrometers.

It is preferable for the collection bus of such a device to have aminimum width of around 1.5 millimeters and a thickness of around 50micrometers.

Such a device can advantageously be a solar cell.

A plurality of solar cells can be combined to form a module, whereinsaid cells are connected in series and/or in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention can be better understood on reading the description ofembodiment examples given purely for indicative purposes and which arein no way limiting, in reference to the appended drawings, in which:

FIG. 1 shows a cross-section of an example of a semiconductor deviceaccording to this invention, in which the collection fingers and thecollection buses are produced according to a metallization method, alsoaccording to this invention;

FIG. 2A shows an example of pressing means used during the sintering ofthe collection fingers;

FIG. 2B shows an example of pressing means used during the sintering ofthe collection buses;

FIG. 2C shows an example of pressing means used during the sintering ofthe metallization located on the rear surface of the semiconductordevice according to this invention;

FIG. 3 shows a top view of the set of collection fingers and collectionbuses, produced according to a metallization method, according to thisinvention;

FIG. 4 shows a top view of a module made of a plurality of solar cellsconnected to one another, also according to this invention.

Identical, similar or equivalent parts of the various figures describedbelow have the same numeric references for the sake of consistencybetween figures.

The various parts shown in the figures are not necessarily shownaccording to a uniform scale, so as to make it easier to read thefigures.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference is made to FIG. 1, which shows a cross-section of an exampleof a semiconductor device 100 with a heterojunction, according to thisinvention, comprising a set of collection fingers 8.1 to 8.n visible inFIG. 3) and at least one collection bus 16 a, 16 b (collection bus 16 bvisible in FIG. 3) produced according to a metallization method, alsoaccording to this invention. In FIG. 1, only two collection fingers 8.iand 8.i+1 belonging to the set of collection fingers 8.1 to 8.n areshown. In this example, the semiconductor device 100 is a solar cell.

The semiconductor device 100 comprising a crystalline semiconductorsubstrate 1 with a first type of conductivity is shown. Thesemiconductor substrate 1 ia for example, N-type thin-layer silicon. Itis not necessary to use an extremely high-quality silicon because, giventhe absence of strong thermal constraints during the semiconductordevice 100 production process, the lifetime of the current carriers ofthe silicon will not be altered. The thickness of the semiconductorsubstrate 1 can be between 10 micrometers and several hundredmicrometers.

The semiconductor substrate 1 comprises a first surface 3 that, in thisexample, is on the side of a front surface 17 of the solar cell. It isthis front surface 17 that is exposed to light. The semiconductorsubstrate 1 comprises a second surface 2 opposite the first surface 3.This second surface 2 is therefore on the side of the rear surface ofthe solar cell.

The semiconductor substrate 1 comprises, on its first surface 3, anamorphous semiconductor layer 4. The amorphous semiconductor layer 4 is,for example, intrinsic. In another embodiment, this amorphoussemiconductor layer 4 can be gradually doped with a second type ofconductivity, opposite the first type of conductivity. It is alsopossible for this layer 4 to be an intrinsic microcrystallinesemiconductor, or for the semiconductor device 100 not to comprise thissemiconductor layer 4. The deposition this amorphous semiconductor layeris performed, for example, by a plasma-enhanced chemical vapourdeposition technique (PECVD).

An amorphous semiconductor layer 5 with a second type of conductivity,i.e. P, is stacked on top of said amorphous semiconductor layer 4. Thesemiconductor used to produce amorphous layers 4 and 5 is, for example,thin-layer silicon. The semiconductor device 100 thus produced comprisesa heterojunction, formed by the semi-conductor substrate 1 and theamorphous semiconductor layer 5. The deposition of the amorphoussemiconductor layer 5 is performed, for example, by PECVD. The thicknessof the amorphous semiconductor layers 4 and 5 is around 75 nanometers.

A conductive transparent oxide layer 6, for example, indium and tinoxide, is located on the amorphous semiconductor layer 5. Thisconductive transparent oxide layer 6 is produced, for example, bycathode sputtering. Its thickness is around 80 nanometers.

The semiconductor device 100 comprises, for example, on the secondsurface 2 of the semiconductor substrate 1, a metal layer 7. This metallayer 7, for example based on silver, is one of the electrodes of thesemiconductor device 100. The semiconductor device 100 can comprise, onthe second surface 2 of the semiconductor substrate 1, a structure thatis different from the metal layer 7. For example, the semiconductordevice 100 can comprise, on its surface 2, the same elements as thoselocated on its surface 3, except for the amorphous semiconductor layerdoped with the second type of conductivity, which would then be dopedwith the first type of conductivity.

We will now describe the method for metallization of the semiconductordevice 100, also according to this invention.

A set of collection fingers 8.1 to 8.n is metallized on the conductivetransparent oxide layer 6. The collection fingers 8.1 to 8.n areproduced, for example, by serigraphy, with the so-called“low-temperature” paste. They have a width of around 100 micrometers anda metallization height of between 20 and 40 micrometers. They each havea metal height substantially identical to within 1 micrometer, and areregularly spaced apart from one another by a distance of around 2millimeters. This arrangement makes it possible to obtain a homogeneouscollection of the current. The number n of collection finger 8.1 to 8.nis therefore dependent on the dimensions of the semiconductor device100. This number must be sufficient for the series resistance of thesemiconductor device 100 not to be too high. The collection finger 8.1to 8.n are arranged so as to be parallel to one another, and are made ofa material based on aluminium or a noble metal, such as silver, forexample.

A sintering operation is then performed, at a temperature below atemperature that would damage the semiconductor device 100, on thecollection fingers 8.1 to 8.n by pressing said collection finger 8.1 to8.n. The pressing is performed, or example, at a temperature betweenaround room temperature and 400° C. and at a pressure between around 10⁶Pa and 2×10⁸ Pa, making it possible to sinter the collection fingers 8.1to 8.n. The temperature of 400° C. is approximately the maximumtemperature because, above it, there would be damage to thesemiconductor device 100, and in particular the amorphous semiconductor.This sintering makes it possible to reduce the resistivity from around5×10⁻⁵ ohm-cm to around 5×10⁻⁶ ohm-cm. This resistivity obtained makesis possible to have a very good peaking factor, greater than 0.75.

FIG. 2A shows an example of pressing means used for the sintering of theset of collection fingers 8.1 to 8.n. In this FIG. 2A, four collectionfingers 8.i−1, 8.i, 8.i+1, 8.i +2 belonging to the set of collectionfinger 8.1 to 8.n are shown. A press 10, for example hydraulic orpneumatic, exerts a pressure on the collection fingers 8.i−1, 8.i,8.i+1, 8.i+2. The semiconductor device 100 is in contact with a support11. This support 11 resists the pressure exerted by the press 10 withoutbeing deformed or moving. The press 10 is not directly in contact withthe semiconductor device 100. Means for uniform pressing 13, 14 are, forexample, a rubber or plastic damper 13 and a plate 14, for example ofsilicon, making it possible to uniformly distribute the pressure exertedby the press 10 on the semiconductor device 100. Protective means 15 a,15 b for the semiconductor device 100 can also be provided between thepress 10 and the semiconductor device 100, and between the support 11and the semiconductor device 100. These protective means 15 a, 15 b makeit possible not to place the semiconductor device 100 in direct contactwith the plate 14 and the support 11, thus preventing impurities frombeing pressed against the semiconductor device 100. These protectivemeans 15 a, 15 b can be, for example polyethylene terephthalate filmsthat are changed for each pressing of a semiconductor device 100.

After the sintering of the collection fingers 8.1 to 8.n, at least onecollection bus 16 a, 16 b is metallized on the collection finger 8.1 to8.n. Again, the number of collection buses is dependent on thedimensions of the semiconductor device 100. The number of collectionbuses must be adapted according to the width of the semiconductor device100. In FIG. 3, two collection buses 16 a, 16 b are metallized on thecollection finger 8.1 to 8.n. These collection buses 16 a, 16 b areproduced by serigraphy, with the so-called “low-temperature” serigraphypaste. The collection buses 16 a, 16 b electrically connect thecollection fingers 8.1 to 8.n to one another. These collection buses 16a, 16 b have a width greater than that of the collection finger 8.1 to8.n. It is at least around 1.5 millimeters. Their metallization heightis around 50 micrometers. The collection buses 16 a, 16 bare positionedso as to be substantially perpendicular to the collection finger 8.1 to8.n. Like the collection fingers 8.1 to 8.n, the collection buses 16 a,16 b are made of a material based on aluminium or a noble metal, such assilver, for example.

In a device produced according to the invention, as the currentcirculating in the collection buses is greater than that circulating inthe collection fingers, the collection buses are wider (for examplearound 2 millimeters) than the collection fingers (for example around100 micrometers). The collection buses are serigraphically printed withmasks of different types, coarser than those used for the serigraphy ofthe collection fingers, requiring a finer definition. These masksgenerate thicker metallizations for the collection buses (for examplebetween around 50 and 100 micrometers) than for the collection fingers(for example between around 20 and 40 micrometers). If a sinteringoperation is performed after the metallization of the collection buses,the areas of the collection fingers located near the collection busesare not subjected to the same pressing as the areas of the collectionfingers farther from the collection buses. The method according to theinvention makes it possible to improve the adhesion and the resistivityof the metallizations with respect to the prior art.

After the metallization of the collection buses 16 a, 16 b, it is alsopossible to perform a sintering operation, at a temperature below atemperature that would damage the semiconductor device 100, on thecollection buses 16 a, 16 b by pressing said collection buses 16 a, 16b. This pressing is shown in FIG. 2B. The pressing is performed underthe same temperature and pressure conditions as the pressing of thecollection finger 8.1 to 8.n. In FIG. 2B, a single collection bus 16 ais shown. This sintering makes it possible to reduce the resistivity ofthe serigraphy paste forming the collection buses 16 a, 16 b. The press10, the support 11, the damper 13, the plate 14 and the protective means15 a, 15 b are identical to those of FIG. 2A.

The method can also comprise a step of sintering, at a temperature belowa temperature that would damage the semiconductor device 100, on themetallization 7 located on the surface 2 opposite the front surface 17of the semiconductor device 100. This additional step is shown in FIG.2C. Again, the temperature and pressure conditions, the press 10, thesupport 11, the damper 13, the plate 14 and the protective means 15 a,15 b are identical to those of FIG. 2A. This sintering can be performedalone, i.e. after the sintering of the collection fingers 8.1 to 8.n andoptionally of the collection bus 16 a, 16 b. It can also be performedsimultaneously with the sintering of the collection finger 8.1 to 8.n orof the collection buses 16 a, 16 b, with these two sintering operationsbeing combined in a single pressing step of the semiconductor device100. In this case, the semiconductor device 100 then comprises only thecollection finger 8.1 to 8.n not sintered, or the collection finger 8.1to 8.n already sintered and the collection buses 16 a, 16 b notsintered, and, on the surface 2, the metallization 7 to be sintered.Thus, in the pressing operation, the pressure exerted makes it possibleto sinter the metallizations 7 and 8.1 to 8.n, or 7 and 16 a, 16 b,located on two opposite surfaces 17, 2 of the semiconductor device 100.

Although a number or embodiments of this invention have been describedin detail, it should be understood that various changes andmodifications can be made without going beyond the scope of theinvention. This metallization is also advantageously applied tosemiconductor devices that cannot, for some reason, be subjected totemperatures above around 400° C., such as, for example, devicescomprising plastic materials.

A number of semiconductor devices according to the invention can beproduced at the same time on the substrate 1, and the unitary devicescan then be electrically connected to one another by their collectionbuses so as to obtain a module of solar cells 20, as shown in FIG. 4. Inthe example of FIG. 4, six solar cells 21 a to 21 f make up the solarmodule 20. The collection buses 16 a, 16 b of the solar cells 21 a, 21b, 21 c are connected in series as are the collection buses 16 a, 16 bof the solar cells 21 d, 21 e, 21 f. The collection buses 16 a, 16 b ofthe solar cells 21 c, 21 f are then connected in parallel to obtain anelectrode 22 of the solar module 20. The electrodes located on the rearsurfaces of the solar cells 21 a to 21 f are also connected to oneanother in a manner identical those located on the front surface.

1. A method for metallization of a semiconductor device, comprising, in order: a) metallizing, by serigraphy, a set of collection fingers with a low-temperature serigraphy paste on at least a front surface of the semiconductor device; b) sintering, at a temperature below a temperature that would damage the semiconductor device, the serigraphy paste forming the set of metallized collection fingers, by performing a pressing operation on the collection fingers with a press; and c) metallizing, by serigraphy, at least one collection bus on the set of metallized collection fingers, electrically connecting the collection fingers to one another, with a low-temperature serigraphy paste.
 2. A method according to claim 1, the semiconductor device comprising a heterojunction.
 3. A method according to claim 1, further comprising, after c), d) sintering, at a temperature below a temperature that would damage the semiconductor device, the serigraphy paste forming the collection bus by a pressing operation on the collection bus performed by the press.
 4. A method according to claim 1, further comprising sintering, at a temperature below a temperature that would damage the semiconductor device, a metallization located on a surface of the semiconductor device opposite the front surface by a pressing operation on the metallization performed by the press.
 5. A method according to claim 1, the press being a hydraulic or a pneumatic press.
 6. A method according to claim 1, the pressing operation being performed at a temperature between around room temperature and 400° C.
 7. A method according to claim 1, the pressing operation being performed at a pressure level between around 10⁶ Pa and 2×10⁸ Pa.
 8. A method according to claim 1, the semiconductor device being placed between the press and a support, during the pressing, and means for protecting the semiconductor device being inserted between the semiconductor device and the press, and between the semiconductor device and the support.
 9. A method according to claim 8, the protective means comprising polyethylene terephthalate films.
 10. A method according to claim 1, wherein means for uniform pressing are inserted between the semiconductor device and the press.
 11. A method according to claim 10, the uniform pressing means including one of a damper, a damper made of rubber or a plastic material, a plate, or a plate made of silicon.
 12. A method according to claim 1, the collection fingers being arranged so as to be parallel to one another.
 13. A method according to claim 1, the collection fingers being regularly spaced apart from one another.
 14. A method according to claim 1, the collection bus being arranged so as to be substantially perpendicular to the set of collection fingers.
 15. A method according to claim 1, the collection fingers and the collection bus being produced with an aluminium-based material, or a noble, or silver.
 16. A semiconductor device comprising collection fingers and at least one collection bus, the collection fingers and the collection bus being made according to the method of claim
 1. 17. A semiconductor device according to claim 16, the collection fingers having a width of around 100 micrometers and a thickness between around 20 micrometers and 40 micrometers.
 18. A semiconductor device according to claim 16, the collection bus having a minimum width of around 1.5 millimeters and a thickness of around 50 micrometers.
 19. A semiconductor device according to claim 16, the semiconductor device being a solar cell.
 20. A module of solar cells, comprising a plurality of solar cells according to claim 19, connected in series and/or in parallel. 